1. Field of the Invention
The invention relates to a readback apparatus which plays back a recording signal from a disc medium.
2. Description of the Related Art
For example, in a magnetic recording and readback apparatus, the signal-to-noise ratio (SNR) considerably deteriorates due to the contact of a recording and read head (to be simply referred to as a head hereinafter) to a disc medium, loss of magnetic substances on the disc medium, and poor alignment precision of the head, recorded data causes burst errors, and its readback becomes impossible.
Of these burst errors, a burst error caused when the head has contacted the disc medium has a feature in that large DC components are superposed on a readback signal when the head has contacted the disc medium, compared to normal readback. By exploiting this feature, a conventional magnetic recording and readback apparatus digitizes a signal read from the disc medium by the head, and detects the DC components of the digitized signal, thus detecting the position of occurrence of that burst error. As a result, a compensation using error correction processing can be applied to certain burst errors.
In the conventional magnetic recording and readback apparatus, an analog readback signal read from a disc medium (magnetic recording medium) by the head is input to an analog-to-digital converter and digitized. At this time, when the head contacts the disc medium and large DC components are superposed on the signal, if an analog signal output from the head in correspondence with a sample value after digitization exceeds a maximum input value to the analog-to-digital converter, the digital output after analog-to-digital conversion is saturated at a maximum output value of the analog-to-digital converter, and a readback waveform is lost.
A burst detector detects if the waveform of the digital readback signal output from the analog-to-digital converter is saturated at the predetermined maximum output value of the analog-to-digital converter. When the burst detector detects burst interference in which the readback signal after digitization is saturated at the maximum output value, a burst position detector detects the position of occurrence of an error due to the burst interference.
The readback waveform which is lost due to the burst interference is input to a finite impulse response (FIR) equalizer, which equalizes the waveform to an arbitrary partial response (PR) target (e.g., a PR target such as PR(1, 2, 2, 2, 1) or the like). Furthermore, when a Viterbi equalizer having a state transition of the PR target executes equalization processing again, readback data which is hard-decided to “0” and “1” and has suffered the burst error is obtained. The Viterbi equalizer executes Viterbi equalization using, as likelihood information, the distances between the readback signal equalized to the PR target and signal points defined by the PR target, thus obtaining readback data which is hard-decided to “0” and “1”.
An error correcting decoder executes error correcting decoding processing using the hard-decided data which is obtained from the Viterbi equalizer and includes the burst error, and the burst position information detected from the readback signal by the burst position detector.
The error correcting decoder corrects the error included in the data after Viterbi equalization using error correcting codes such as Reed-Solomon (RS) codes or the like that requires hard-decided data upon decoding with respect to the hard-decided data obtained by Viterbi equalization, thereby obtaining user data.
With the aforementioned processing operations, even when a readback signal includes burst errors/losses caused when the head has contacted the disc medium, errors in readback data can be corrected. However, the aforementioned burst position detection method detects interference from an adjacent track, and appropriate processing cannot be applied.
In a magnetic recording and readback apparatus which plays back recorded data from a disc medium having neighboring data tracks, when a recording signal for an adjacent track is overwritten on that on a target track due to any offtracking of the head, or when the head position is displaced to an adjacent track upon readback to simultaneously readback signals on the target track and the adjacent track, the signal on the adjacent track becomes burst interference noise with respect to the signal obtained by reading back the target track, but no DC components are generated unlike in the aforementioned case. Therefore, since the signal after analog-to-digital conversion does not have any distinctive feature, and a burst error position cannot be detected, the error rate characteristics upon readback deteriorate considerably.
At this time, when error correcting codes using likelihood information are used upon error correcting decoding, the reliability of likelihood information of a part that has suffered burst interference from the adjacent track in the readback data becomes considerably low. For this reason, the potential correction capability of error correcting codes cannot be exploited, and the error rate of readback data after error correction deteriorates undesirably.
A reference (JP-A 2005-166089 (KOKAI)) discloses a technique for improving the decoding function for burst errors caused when the head has contacted the disc medium. However, the technique disclosed in this reference does not consider any burst error from an adjacent track. This reference cannot improve an error rate upon occurrence of interference from an adjacent track.
Since the conventional magnetic recording and readback apparatus cannot detect any interference noise from an adjacent track due to a offtracking of the head, it cannot accurately calculate likelihood information in a part that has suffered interference in the readback data. For this reason, when error correcting codes using likelihood information are used upon error correcting decoding, sufficient error correction capability cannot be exploited under the influence of likelihood information with low reliability of the interference part.